1. Field of the Invention
Embodiments of the invention relate generally to the field of transport of CATV and digital signals over optical fiber. More particularly, an embodiment of the invention relates to DWDM transport of CATV and digital signals over optical fiber in low-dispersion spectral regions.
2. Discussion of the Related Art
Bandwidth Race
CATV Multiple System Operators (MSOs) compete with Incumbent Local Exchange Carriers (ILECs) to offer the highest bandwidth connections to High Speed Data (HSD) subscribers. MSOs were first to offer HSD services and, consequently, enjoyed the lion's share of this market until recently. However, ILECs have recently found success in increasing their share of the HSD market by offering prices below those charged by MSOs. In turn, MSOs have responded, not by lowering their prices, but by offering higher bandwidths for the same price.
MSOs have historically been able to offer higher bandwidths on their hybrid fiber-coax (HFC) networks than ILECs with their traditional digital subscriber loop (DSL) services. Competing on the basis of bandwidth, rather than price, MSOs initially increased downstream speeds to 3 Mb/s, then 5 Mb/s and, more recently, to 10 Mb/s or higher for the same price they previously charged for only 1.5 Mb/s service.
So far, this has been a good strategy for MSOs. However, even higher speeds are now being planned by ILECs using improved variants of DSL such as ADSL2+. In an effort to maintain their market share, MSOs are responding by upgrading their networks to maintain their bandwidth superiority.
Bandwidth Upgrades Limited by Fiber Scarcity
The fiber node (FN) is the demarcation point between the fiber network and the coax network in an HFC network. In comparison with the traditional coax plant, which is limited to frequencies up to 1 GHz, optical fiber provides enormous bandwidth.
In HFC networks, fiber nodes have a nominal fiber service area of anywhere from 500 homes-passed to as many as 2,000 homes-passed. MSOs have been able to increase the bandwidth offered per subscriber by replacing older nodes with newer, segmentable nodes with multiple optical downstream receivers and upstream lasers, effectively reducing the fiber service area.
The next step to increase bandwidth is to either locate a second node next to the original node or to augment the existing node with one or more nodes closer to the subscribers. However, both of these methods exacerbate the problem of fiber scarcity as they require additional fibers to support the new nodes. Even MSOs with the foresight to place several fibers to each node are now finding that the available fibers are not sufficient for the number of new nodes that are required.
Moreover, recent architectures deployed nodes that serve 100 homes-passed but several nodes (often called a node cluster) share forward and reverse fiber to serve the same amount of homes-passed (500 to 2000). In the future, these same fibers will have to support cluster segmentation into individual nodes.
C-Band and L-Band DWDM
In the digital world, fiber scarcity has been solved through the use of dense wavelength-division-multiplexing (DWDM) in the 1550 nm window of optical fibers. In this technique, up to 128 (or more) high-speed digital channels are transported over a single fiber using lasers separated in the wavelength domain in the range of 1530 nm to 1565 nm, commonly referred to as the C-Band. The use of low-noise, high-power erbium-doped fiber amplifiers (EDFAs) optimized for use in the C-Band makes long-distance C-Band DWDM possible. The more recent development of EDFAs that operate in the L-Band (1565-1625 nm) allows the use of L-Band DWDM in digital networks. These digital networks operate over a variety of fiber types of various vintage. Standard transmission fiber such as Corning SMF-28 or its equivalent, with a zero dispersion wavelength in the O-Band (1260 nm to 1360 nm), is widely used for shorter links. Longer or more technically challenging links may employ not only standard transmission fiber, but other types of fiber such as dispersion-shifted fiber, large effective area fiber, or mixtures of fiber types in order to manage dispersion or other impairments.
C-Band DWDM technology has been successfully applied to HFC networks. Such architectures employ a broadcast layer that typically consists of an externally modulated broadcast transmitter carrying sub-carrier-multiplexed (SCM) analog video and QAM-modulated RF signals together with a narrowcast overlay consisting of directly modulated lasers transporting QAM-modulated RF sub-carriers transporting video, voice, and data. In the access part of an HFC network, the use of different transmitters for different signals is largely driven by the characteristics of standard transmission fiber with its zero dispersion wavelength located in the O-Band. In this fiber type, signal impairments caused by linear and nonlinear fiber effects, as described below, must be carefully managed. Although very effective technically, such an architecture is best suited for applications where longer distances and/or large numbers of wavelengths per fiber are required. It is an expensive solution for applications where shorter transport distances and/or lower numbers of wavelengths per fiber are needed. Moreover, this architecture requires some level of management for the RF frequencies of the SCM signals carried on the narrowcast transmitters.
CWDM
Another wavelength-division multiplexing (WDM) technique that is now being deployed is coarse-wavelength-division-multiplexing (CWDM). In this technique, widely-spaced (20 nm) optical signals in the range of 1270 nm to 1610 nm (a total of 18 channels) are employed. This technique has been successfully employed in HFC networks in the upstream links from optical nodes. The use of CWDM for transport of CATV services downstream, especially for the purpose of node segmentation, is currently attracting much attention.
Problems Due to Linear and Nonlinear Fiber Effects
All of the WDM techniques described above (C-Band DWDM, L-Band DWDM and CWDM) are presently employed for digital transport, but their application to the transport of CATV signals in the form of AM-VSB video and M-QAM data is complicated by two types of impairments: crosstalk due to the third-order nonlinear-optical susceptibility of the fiber and distortion caused by dispersion acting on the optical signals generated by directly-modulated DFB lasers.
The third-order nonlinear optical susceptibility of the optical fiber leads to crosstalk between optical carriers through the effects of Stimulated Raman Scattering (SRS), self-phase modulation (SPM), cross-phase modulation (XPM), and the more general case of four-wave-mixing (4WM).
The light emitted by a directly-modulated DFB laser exhibits frequency chirp—the wavelength of the light emitted by the laser varies with the output power of the laser. Chromatic dispersion of the optical fiber acts upon this frequency-chirped light to create distortion in the sub-carrier multiplexed signals of typical analog CATV video transport, primarily composite second-order (CSO) distortion.
SRS-Induced Crosstalk
Although SPM can be a problem in single-channel systems, the main sources of crosstalk in WDM systems are XPM (cross-phase modulation) and SRS (simulated Raman scattering). Of these, SRS-induced crosstalk dominates over XPM crosstalk for channel separation greater than several nm, such as in CWDM systems. SRS crosstalk is a function of the RF frequency, optical power and fiber length. For example, the severity of SRS crosstalk as a function of the wavelength separation of two sub-carrier-multiplexed lightwaves at the CATV frequencies of 55 MHz and 499 MHz are shown in FIGS. 1 and 2 for typical CATV power levels and fiber distances. This crosstalk severely limits the number of downstream CWDM wavelengths and their launch power into the fiber.
CSO Induced by Fiber Dispersion and Laser Chirp
Another severe impediment to the use of CWDM for CATV transport is the CSO induced by fiber dispersion and laser chirp. This problem can be avoided through the use of externally-modulated laser transmitters with negligible chirp. Indeed, this type of transmitter is used to transport CATV signals over a 1550 nm system. However, the cost of externally-modulated laser transmitters is an order of magnitude greater than for directly-modulated CATV laser transmitters. Consequently, the use of externally-modulated laser transmitters in applications such as the segmentation of nodes served by directly-modulated 1310 nm laser transmitters cannot meet the cost constraints of such applications.
For directly-modulated DFB laser transmitters, the worst case CSO for analog channels in the broadcast band (55 MHz-550 MHz) occurs at the upper edge of the band and is plotted in FIG. 3 for various values of laser chirp. Note that since this is due to a linear fiber effect, the induced CSO is independent of optical power in the fiber. However, the dispersion-induced CSO depends on the total dispersion in the fiber (especially SMF-28 and equivalent fibers), which is usually greater if the wavelength (optical frequency) is further away from the zero dispersion wavelength of the fiber.
It can be seen that dispersion-induced CSO increases very rapidly as one moves away from the zero-dispersion wavelength of conventional optical fiber (around 1311 nm). Consequently, CATV transport using CWDM can only be accomplished with a few CWDM wavelengths and even this requires severe constraints on the laser chirp.
Other remedies that are currently being investigated include the use of low-chirp, directly-modulated External Cavity Lasers (ECL). However, a problem with these other approaches is that external cavity lasers, and the predistortion and SBS-suppression circuitry required, are significantly more expensive than traditional DFB lasers.
What is needed is an cost-effective approach that increases bandwidth in fiber scare networks while simultaneously minimizing distortion and cross talk.